In literature there are lot of types of pumps and pump impellers for this purpose described, all however having certain disadvantages. Above all this concerns problems with clogging and low efficiency.
Sewage water contains a lot of different types of pollutants, the amount and structure of which depend on the season and type of area from which the water emanates. In cities plastic material, hygiene articles, textile etc are common, while industrial areas may produce wearing particles. Experience shows that the worst problems are rags and the like which stick to the leading edges of the vanes and become wound around the impeller hub. Such incidents cause frequent service intervals and a reduced efficiency.
In agriculture and pulp industry different kinds of special pumps are used, which should manage straw, grass, leaves and other types of organic material. For this purpose the leading edges of the vanes are swept backwards in order to cause the pollutants to be fed outwards to the periphery instead of getting stuck to the edges. Different types of disintegration means are often used for cutting the material and making the flow more easy. Examples are shown in SE-435 952, SE-375 831 and U.S. Pat. No. 4,347,035.
As pollutants in sewage water are of other types more difficult to master and as the operation times for sewage water pumps normally are much longer, the above mentioned special pumps do not fullfil the requirements when pumping sewage water, neither from a reliability nor from an efficiency point of view.
A sewage water pump quite often operates up to 12 hours a day which means that the energy consumption depends a lot on the total efficiency of the pump.
Tests have proven that it is possible to improve efficiency by up to 50% for a sewage pump according to the invention as compared with known sewage pumps. As the life cycle cost for an electrically driven pump normally is totally dominated by the energy cost (c:a 80%), it is evident that such a dramatic increase will be extremely important.
In literature the designs of the pump impellers are described very generally, especially as regards the sweep of the leading edges. An unambigous definition of said sweep does not exist.
Tests have shown that the design of the sweep angle distribution on the leading edges is very important in order to obtain the necessary self cleaning ability of the pump impeller. The nature of the pollutants also calls for different sweep angles in order to provide a good function.
Literature does not give any information about what is needed in order to obtain a gliding, transport, of pollutants outwards in a radial direction along the leading edges of the vanes. What is mentioned is in general that the edges shall be obtuse-angled, swept backwards etc. See SE-435 952.
When smaller pollutantans such as grass and other organic material are pumped, relatively small angles may be sufficient in order to obtain the radial transport and also to disintegrate the pollutants in the slot between pump impeller and the surrounding housing. In practice disintegration is obtained by the particles being cut through contact with the impeller and the housing when the former rotates having a periphery velocity of 10 to 25 m/s. This cutting process is improved by the surfaces being provided with cutting devices, slots or the like. Compare SE-435 952. Such pumps are used for transport of pulp, manure etc.
When designing a pump impeller having vane leading edges swept backwards in order to obtain a self cleaning, a conflict arises between the distribution of the sweep angle, performance and other design parameters. In general it is true that an increased sweep angle means a less risk for clogging, but at the same time the efficiency decreases.